In recent years, a transistor including an oxide semiconductor has attracted attention. An oxide semiconductor can be formed by a sputtering method or the like, and thus can be used for a semiconductor of a transistor in a large display device. In addition, the transistor including an oxide semiconductor is advantageous in reducing capital investment because part of production equipment for a transistor including amorphous silicon can be retrofitted and utilized.
It is known that a transistor including an oxide semiconductor has an extremely low leakage current in an off state. For example, a low-power-consumption CPU utilizing a characteristic of low leakage current of the transistor including an oxide semiconductor is disclosed (see Patent Document 1).
Furthermore, a method for manufacturing a transistor including an oxide semiconductor in which a gate electrode is embedded in an opening portion is disclosed (see Patent Documents 2 and 3).